Rotary winged aircraft or helicopters as they are more commonly known comprise a generally vertical rotor or must having a lower end held within a fuselage. They also include a plurality of wings or blades which rotate with the rotor. These rotating wings cause the aircraft to travel vertically, forward, backward and sideways. The action of the blades upon the air result in a movement downward therefrom of a turbulent column of air which engulfs the fuselage as well as any Pitot and static tubes which are placed thereon to give airspeed indications.
It is also known in the art of sensing aircraft speed and direction to place a pressure sensor at the end of a rotating wing and to interpret the pressure variations which occur as a result of aircraft movement through an air mass to provide airspeed and direction signals. For example, a U.S. Pat. No. 3,332,282 of D. F. Daw discloses a pressure sensor probe at the tip of one of the helicopter's rotor blades. As the blade rotates, the amplitude of the cyclic variation of the blade tip pressure per revolution is a function of the translational speed of the helicopter. The cyclic variation is converted into an electrical signal which is passed through an inductive coupling using a pair of coils. By positioning the coils at right angles with respect to one another and in a known position with respect to the longitudinal axis of the aircraft, any component of the translation speed may be determined and from component measurements, the resultant translational direction with respect to the aircraft's axis can be derived.
A more recent patent of Onksen et al. U.S. Pat. No. 4,360,888 discloses an omni-directional airspeed system which calculates airspeed from a differential pressure signal. The differential pressure signal is indicative of the pressure difference between two rotating pitot-type sensors which are mounted at the ends of the hollow tubular arms. At airspeed other than zero, the velocity of the air through the sensors varies sinusoidally, with maximum difference when the sensor arms are aligned perpendicular to the wind. At that instant, the velocity of the sensor advancing into the wind is equal to the tip speed plus the airspeed, and the velocity of the air in the sensor retreating from the wind is equal to the tip speed minus the airspeed. The resultant pressure in the two hollow tubes are then different and the transducer outputs a voltage proportional to the differential pressure and proportional airspeed. Then, when the arms are aligned parallel to the wind, the wind velocity in the sensors are equal and the differential pressure output equals zero. The resultant wave form from the differential transducer is sinusoidal, with amplitude related to airspeed and phase related to direction.
An improvement in the aforementioned devices is disclosed in the U.S. Pat. No. 4,893,261 of Flint III, et al. As disclosed therein, aircraft speed and direction are determined by sampling the sinusoidal-like pressure variations at the end of a rotor and performing a Fourier analysis on the pressure samples.
It is presently believed that there is a relatively large commercial market for an improved airspeed and direction indicating system for rotary winged aircraft. It is believed that there is a demand for such systems which eliminates pressure sensor and pressure transducers, are highly reliable, relatively easy to install and service and which can be manufactured and sold at a competitive price. It is also believed that the airspeed and direction indicating system in accordance with the present invention provides all of the aforementioned benefits and more as will become evidence from the following description.